The invention relates to a device of fitted variable gain analog-digital conversion. The conversion device preferably converts digitally signals produced by a photosensitive cell of an image sensor. The photosensitive cell is made up of a matrix of pixels.
The conversion device therefore comprises at least one N-bit first converter receiving a voltage or current signal of one pixel and at least one M-bit second converter connected to the first converter, the first and second converters converting the voltage or current level of the pixel to N+M bits. The voltage or current level of the signal produced by each pixel is dependent on a level of light picked up by the pixel in a particular voltage or current dynamic range of the sensor.
The first converter of the device comprises comparison means for comparing the voltage or current level of the pixel with one or more voltage or current thresholds. These voltage or current thresholds delimit successive voltage or current ranges within the dynamic range. Said successive voltage or current ranges within the dynamic range are used to define the illumination of a pixel, ranging from a weakly illuminated pixel to a strong illuminated pixel. The first converter supplies a N-bit binary word whose value relates to the voltage or current range in which the voltage or current level of the pixel is situated.
The variable gain conversion means conversion using a number of bits greater than the number of bits retained for each pixel after conversion. In this way it is possible to apply digital amplification as a function of the level of illumination of the pixels.
The invention relates equally to an image sensor comprising in particular a pixel matrix photosensitive cell, an analog-digital conversion device connected to the cell, an illumination averaging unit connected to the conversion device, and a scale adapter connected to the conversion device and to the averaging unit.
The invention also relates to an analog-digital conversion method for operating the analog-digital conversion device.
To capture an image, a photosensitive cell generally comprises a matrix of pixels in order to supply each signal converted into a voltage representing the number of photons captured, for example. The higher the number of photons, the greater the voltage difference produced. A digital image is usually quantised on 8 bits, i.e. with 256 possible levels. In the case of a colour image, each primary (red, green, blue) component is coded on 8 bits.
In this connection, it is as well to remember that each pixel comprises the capacitance of a junction, such as that of a photodiode, for capturing photons, in particular with a well of 100 000 electrons. In normal operation, this capacitance (photodiode) is reverse biased to a given voltage from 0 to 2 V, for example.
In an image active-pixel sensor (APS) implemented in a CMOS technology, the photons discharge a capacitor to generate electron-hole pairs. The electron-hole pairs are collected by the opposite electrodes of the capacitor and consequently reduce the voltage difference across the capacitor. As this voltage difference decreases with illumination, the polarity of the signal is reversed, i.e. there is a high voltage when the pixel is strongly illuminated and a low voltage in the event of weak illumination. Thus the dynamic range of the sensor voltage is less than the bias voltage of the capacitor, for example equal to 1.5 V. This condition is not limiting, however.
To convert the voltage signals produced by the pixels, the signals must generally be amplified. The amplification depends on the level of illumination of the pixels of the captured image. To amplify the signals, one option is to pre-amplify each pixel signal before analog-digital conversion, for example, as shown diagrammatically in FIG. 1a. To do this in the image sensor, a certain number of variable gain amplifiers 101 are each connected to the output of a respective pixel (not shown) to receive a converted voltage Vpix corresponding to the captured illumination, for example. The variable gain amplifier 101 for each pixel amplifies the substantially constant voltage Vpix by an amplification factor Ax to provide an amplified output voltage signal AxVpix. The amplified signal is then converted digitally in a standard 8-bit AD converter 100 to produce an 8-bit binary word Sn.
The amplification factor Ax of the amplifiers is adjusted to the dynamic range of the converter after averaging the levels of illumination of some pixels in particular. This averaging is effected by an illumination averaging unit 102 connected in a feedback loop between the converter 100 and the amplifier(s) 101. A control signal S_Ax for adjusting the amplification factor is supplied to the amplifier by the averaging unit.
To fix the amplification factor, it is necessary to effect a plurality of analog-digital conversions in order to reach an optimum state of the average illumination of the image captured by the pixels, which is a drawback. Another drawback with analog amplification of the voltage of each pixel is that this leads to high power consumption, caused in particular by overworking the image sensor. This therefore makes it difficult to use this kind of sensor in a portable object, such as a wristwatch, which is supplied with power by small batteries or accumulators. What is more, it is difficult to connect a plurality of matched analog amplifiers in parallel in the same semiconductor structure to save time converting an image captured by the pixel matrix.
Another solution for amplifying the pixel signals is to employ digital amplification using a variable gain analog-digital converter of an image sensor as represented diagrammatically in FIG. 1b. For this kind of digital amplification, the converted voltage Vpix for each pixel is first digitised using an (8+n)-bit variable gain AD conversion device 110. The binary word produced by the converter 110 is supplied to a scale adapter 111 which is responsible for taking the same eight successive bits from each binary word of (8+n) bits and supplying a binary signal Sn on 8 bits. The choice of the eight successive bits taken from each binary word depends on an illumination average of a subset of pixels of the matrix that has captured an image to be digitised. The illumination average is obtained by means of an illumination averaging unit 112. For example, the illumination averaging unit 112 calculates an average over a plurality of (8+n)-bit binary words from the converter 110 in order to determine which bits are the most representative of the digitised voltage signals Vpix.
Accordingly, with this type of digital amplification, it is possible to defer a decision on the average level of illumination of the image captured by the matrix of pixels, which avoids preliminary exposure control, as is the case with analog amplification. However, with a standard conversion device 110 of this kind, conversion to (8+n) bits is effected under all circumstances of illumination of the pixels, which may be a drawback. Conversion with this accuracy surplus is not always necessary, especially in the event of strong illumination of the pixels of the photosensitive cell, as image sensor noise from the photosensitive cell is greater with strong illumination than with weak illumination. Thus when determining the less significant bits for a strongly illuminated pixel, the converter may convert voltage or current levels lower than the noise of the pixel. This renders this operation superfluous, since it is random, and this is a drawback.
At this connection, one can cite U.S. Pat. No. 4,733,217 which describes a sub-ranging analog to digital conversion device. This conversion device includes a N-bit first coarse converter, which receives a video voltage or current signal, and a M-bit second fine converter connected to the first converter. Said N bits provided by the first converter determine a voltage or current range in which the voltage or current signal is situated within a voltage or current dynamic range. Said voltage or current range is determined within the first converter after signal comparison operations with voltage or current thresholds. A combine element, connected to first and second converters, receives the N bits MSB from the first converter and the M bits LSB from the second converter for supplying a N+M bit binary word.
A drawback of such a conversion device of the herein-above patent is that it is not able to adapt the conversion of the voltage or current signal as a function of the voltage or current level of analog signal to be converted.